Voltage-reference diodes are semiconductor devices which have been widely known and used in the industry for many years. A voltage-reference diode is used in the reverse bias breakdown mode. A voltage-reference is designed so that, once it is in the breakdown mode, the voltage across it is stable over a relatively wide range of currents. It is common practice to refer all voltage-reference diodes as zener diodes, although the later term correctly refers only to a particular type of junction breakdown which may occur in a voltage-reference diode.
A widely recognized problem of voltage-reference diodes is that the breakdown voltage is not stable as the temperature of the junction is varied. An early solution to this problem was to combine a voltage-reference diode with a conventional diode having an equal and opposite temperature coefficient. Typically, the two diodes are separate, discrete devices which are combined in a single package. Such temperature compensated diodes remain a viable product of the semiconductor industry.
There have been attempts to manufacture monolithic, or single chip, temperature compensated voltage-reference diodes. However, these devices have generally involved relatively complex mesa and/or double sided processing which make them relatively expensive.